Ever higher requirements are imposed on an internal combustion engine of a motor vehicle with respect to a reduced consumption of fuel, on the one hand, with a simultaneously low discharge of toxic exhaust gases, on the other hand. This has led to the situation that the air/fuel mixture (emanating from the fuel in the fuel tank), the so-called regeneration gas, is likewise supplied to the combustion process in the engine and is used in this manner. The mass flow and the hydrocarbon concentration of the regeneration gas are determined to consider the regeneration gas when injecting fuel into the combustion chamber. In this way, the fuel quantity, which is contained in the regeneration gas, is computed and thereby can be subtracted from the fuel quantity to be injected. The determination of the hydrocarbon concentration of the regeneration gas takes place for non-direct injection engines with the aid of a lambda probe which is subjected to the exhaust gas generated by the combustion. For this determination, it is necessary to integrate a variable, which is dependent upon the lambda sensor, during the regeneration operation over a longer time interval.
Internal combustion engines wherein fuel is not injected directly into the air intake pipe but is injected directly into the combustion chamber have the advantage that a further reduction of the fuel consumption is possible especially for low loads. In these load ranges, the fuel is injected directly ahead of the ignition and preferably into the region of the spark plug. A mixing with air takes place only to a slight extent and it is for this reason that this defines the so-called stratified operation of the direct-injecting engine. In this stratified operation, the ratio of the fuel and air exhibits a characterizing lambda value having a very high value because of the reduced fuel quantity referred to the inducted air. It is known that for high lambda values of the lambda sensor of this kind, only very imprecise measuring results are supplied so that faults are exhibited by an integration of the above-mentioned quantity dependent upon the lambda sensor. For this reason, a computation of the concentration of the regeneration gas with the aid of the integration method known to date serves no real purpose in stratified operation.